Interpretive Summary: Varroa mites are one of the most important pests of honeybees worldwide and beekeepers are very interested in new ways to treat varroa infestations without using chemicals that contaminate honey and wax. Biopesticides, which use pest diseases to control the pests, are one option, but many diseases of insects and mites are not very specific and might hurt the bees. We used a strain of fungus that we found attacking varroa mites in a commercial apiary in France, and which we have already used in field experiments, to see if the fungus hurt the bee colony by weighing the beehives and by monitoring the adult and larval bees and the honey stocks. We found no negative impact of fungus treatment on the bees at all, and we did find that the fungus caused more mites to fall than usual, as it should. We combined results here with results from other experiments to see how dosage and temperature affect the proportion of infected mites in treated hives. Commercial and hobby beekeepers should benefit from a new, chemical-free way to kill varroa mites.

Technical Abstract:
In 2 field experiments bee colonies in southern France were treated with conidia of an isolate of Beauveria bassiana collected from varroa mites in the region. The main objectives were to evaluate the effect of treatment on colony growth, on total adult bee weights, on the amounts of sealed brood and honey, and on varroa populations. Bee colonies in the 1st experiment were treated with fungal conidia formulated with either wax powder or wheat flour, wheat flour alone, or left as untreated controls. Fungal treatment had a significant positive effect on colony growth. No significant treatment effects were observed 28 days after treatment with respect to total weight of adult bees, or the surface areas of sealed brood or honey, and no infected bees were detected either during colony inspections or in frame photographs. Colonies treated with conidia and wax powder had a significantly higher mite fall than controls overall, but colonies treated with conidia and wheat flour did not. However, the proportion of infected mites in both treatments that included conidia was significantly greater than that in control hives for up to a week after treatment. In the 2nd experiment, conducted in the late fall and early winter, hives were either treated with a mixture of B. bassiana conidia and wax powder, or kept as untreated controls. Fungal treatment did not have a significant effect on mite fall in that experiment, but the proportion of infected mites was significantly higher among treated hives. These results were considered with those of previous studies to examine the relationship between dosage and proportion infected mites, and between ambient temperature and the duration of the infection.